1. Field of the Invention
The invention is related to the field of Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) apparatus and methods. More particularly, the invention relates to apparatus and methods for detecting explosives and flammable liquids.
2. Background Art
The description of the invention and its background are explained herein in the context of screening devices for detecting liquids being carried by passengers into the passenger cabin of an aircraft.
In a response to terror plots, authorities across the world have placed extreme limits on liquid items being taken through passenger security checkpoints onto commercial aircraft. The authorities have banned things such as hair gels, deodorants, ointments, chapped lip remedies, aerosols, etc. from being carried by passengers into the passenger cabin on all commercial airline flights. In light of this, it is obvious that a liquid detection system is very important and should be employed in conjunction with other effective screening devices at passenger checkpoints, throughout the world's airports.
X-ray screening systems and metal detectors are known in the art for detecting contraband that could be physically used as weapons. An example of an x-ray system is described in U.S. Pat. No. 7,092,485 issued to Kravis. Such a system includes combined angular and energy dispersive x-ray scanning to detect the presence of a contraband substance within an interrogation volume of a baggage item. The interrogation volume is illuminated with penetrating, polychromatic x-rays in a primary fan beam from a source such as a tungsten-anode x-ray tube. An energy-dependent absorption correction is determined from measurement of the attenuation of the fan beam at a plurality of different energies. Radiation coherently scattered by substances in the interrogation volume is detected by an energy-resolved x-ray detector operated at a plurality of scattering angles to form a plurality of scattering spectra. Each scattering spectrum is corrected for energy-dependent absorption and the corrected spectra are combined to produce a scattering pattern. The experimental scattering pattern is compared with reference patterns that uniquely characterize known contraband substances. The system and method can locate and identify a wide variety of contraband substances in an accurate, reliable manner. The system provides for automated screening, with the result that vagaries of human performance are virtually eliminated. False alarms and the need for hand inspection are reduced and detection efficacy is increased.
However, neither X-ray devices nor any other technology known in the art can effectively detect liquids and liquid-based explosives compound. Special difficulties are associated with the detection of liquids soaked in cloth which may not be resolved by x-ray scanning techniques.
NMR methods related to plastic explosive detection are described in U.S. Pat. No. 4,514,691 issued to De Los Santos, et al. A method and apparatus disclosed in the '691 patent are directed to baggage inspection. A conveyor system is utilized to transport a bag past a first magnet to achieve an initial nuclear magnetic resonance (NMR) polarization of a selected element. The selected element in the preferred embodiment is hydrogen, it being noted that hydrogen in explosives has a long longitudinal relaxation time (T1) and a short transverse relaxation time (T2). The first magnet provides an initial polarization. The bag continues on the conveyor belt past a second magnet. The second magnet provides the measurement field intensity and a coil contains an interacting RF forming the interrogation pulses. Nuclear magnetic spin echoes are generated and received as a result of the interrogating pulses. First and subsequent second echoes are received and stored. The two signals are subtracted from one another to a null. The NMR response of elemental hydrogen in compounds typified by explosives is accented by subtraction of the two stored signals to form a response indicative of explosive materials. The apparatus includes means for adjusting the magnetic field to accommodate distortions for metal in the baggage.
A method related to plastic explosive detection is described in U.S. Pat. No. 6,194,898 issued to Magnuson et al. A system disclosed in the '898 patent detects a target substance within a class of explosives and narcotics containing quadrupolar nuclei through the use of nuclear quadrupole resonance (NQR). The system applies an RF signal to a coil to excite the substance under test. If the target material is present, an NQR signal will be picked up by the same coil. That signal is compared with known NQR signals in frequency and amplitude. A signal is displayed in an appropriate way if a threshold value of the NQR signal is exceeded. The empty coil is statically tuned by means of adjusting the location or capacitance values, or both, of static tuning capacitors in the coil. The coil is tuned after the specimen is inserted into the coil by means of an auto-tune feature. Effective RFI shielding is provided to prevent external contaminating signals from being detected by the coil and to prevent RF signals from escaping from the scanner. The invention also includes the method for performing tests with the system.
Prior art detecting devices are directed to determining the presence of metals and/or plastic explosives in carry-on baggage. While effective at deterring such materials from being carried onto aircraft, other threats, namely hazardous liquids, have become known. Plastic explosives, for example, are necessary to be carried in large amounts to represent an actual threat to an aircraft. Some sort of metallic detonator must be used for initiation of plastic explosive and would therefore be expected to be found in carry on baggage items subjected to ordinary screening. Liquids, however, may be poured together from small containers into a large one and be ignited or detonated one on board an aircraft. As explained above detection of liquids using techniques known in the art has proven difficult because of the low density contrast with harmless carry on items and the fact that hazardous liquids may be soaked into articles of clothing or separated into smaller containers and distributed throughout a piece of carry on baggage making them difficult to detect.
What is needed is a system for rapidly detecting liquids and characterizing them as safe or hazardous so as to reduce the number of visual inspections of passengers and carry on articles when loading aircraft.